Semiconductor apparatus, method for delaying signal thereof, stacked semiconductor memory apparatus, and method for generating signal thereof

ABSTRACT

The semiconductor apparatus includes a reference delay value check unit configured to receive a source signal and delay the source signal to generate a reference delay signal; a process delay value check unit configured to receive the source signal and delay the source signal to generate a process delay signal; and a signal generation unit configured to receive the reference delay signal and the process delay signal, receive an input signal, and variably delay the input signal based on the reference delay signal and the process delay signal to generate an output signal.

CROSS-REFERENCES TO RELATED APPLICATION

The present application claims priority under 35 U.S.C. §119(a) toKorean Patent Application No. 10-2010-0106880, filed on Oct. 29, 2010,in the Korean Intellectual Property Office, which is incorporated hereinby reference in its entirety as if set forth in full.

BACKGROUND

1. Technical Field

This disclosure relates to a semiconductor apparatus, and moreparticularly, to a stacked semiconductor memory apparatus.

2. Related Art

A three-dimensional arrangement structure having plurality of memorychips stacked therein is used to improve the degree of integration of asemiconductor memory. A semiconductor memory apparatus using thethree-dimensional arrangement structure may be referred to as a stackedsemiconductor memory apparatus.

In the stacked semiconductor memory apparatus, each memory chip can becalled a slice, and slices may be differently coupled to one anotherdepending on the stacking mechanisms of the stacked semiconductor memoryapparatus. Stacking mechanisms of the stacked semiconductor memoryapparatus may include a system in package (SIP) method, a package onpackage (POP) method, a through-silicon via (TSV) method and the like.According to the stacking mechanisms, the slices may be electricallyconnected to one another using balls, wires or bumps. The TSV method hasbeen proposed as stack method solution for overcoming the reduction in atransmission speed due to the distance to a controller, weakness of adata bandwidth, and the deterioration of data transmissioncharacteristics due to various variables on a package.

FIG. 1 is a schematic diagram of a stacked semiconductor memoryapparatus using a typical TSV method. The stacked semiconductor memoryapparatus illustrated in FIG. 1 has a configuration in which a masterchip Master controls a plurality of slave chips Slave. The typicalstacked semiconductor memory apparatus illustrated in FIG. 1 operates asfollows.

When a read or write command is generated from the master chip of thestacked semiconductor memory apparatus, the master chip transmits afirst timing signal AYP to the plurality of slave chips Slave. The firsttiming signal AYP may serve as a source signal of various timing signalsgenerated for a read or write operation by the plurality of slave chipsSlave. The first timing signal AYP may also individually exist in eachof the plurality of slave chips Slave, and one first timing signal AYPmay also be shared by the plurality of slave chips Slave through asingle path (such as a TSV in the TSV method). The number of TSVs in thecontemporary stacked semiconductor memory apparatus using the TSV methodhas been gradually reduced from the consideration of a layout and anavailable area so that the first timing signal AYP illustrated in FIG. 1may be formed of a single signal transmitted through a single path forexample, a TSV) shared by the plurality of slave chips Slave.

After the first timing signal AYP is received, the plurality of slavechips Slave generate various timing signals for a read or writeoperation through respective timing signal generation units 100. Thevarious timing signals will be described later with reference to FIG. 2.Each of the plurality of slave chips Slave generates second timingsignals PIN for a read operation. The second timing signal PIN includessynchronization information required when the plurality of slave chipsSlave transmit data to the master chip. The master chip receives thedata, which is transmitted from the plurality of slave chips Slave, insynchronization with the second timing signal PIN. In the stackedsemiconductor memory apparatus illustrated in FIG. 1, the plurality ofslave chips Slave may share a single path for the second timing signalsPIN transmitted to the master chip. Furthermore, the plurality of slavechips Slave may share a single path for the data transmitted to themaster chip. Accordingly, the second timing signal PIN should beactivated at an accurate timing. In more detail, since the plurality ofslave chips Slave and the master chip share a path for data beingtransmitted and a path for the second timing signal PIN beingtransmitted, it is necessary for each slave chip Slave to accuratelytransmit the data and the second timing signal PIN within the time forusing the path. Here, skew for the second timing signal PIN may beproblematic. Each slave chip Slave generates the second timing signalPIN after a predetermined time passes from the point of time at whichthe first timing signal AYP is triggered. However, the second timingsignal PIN generated by each slave chip Slave may be deviated from atarget point of time due to PVT (process, voltage, temperature)variation. Moreover, since each slave chip Slave may be fabricated fromdifferent wafers rather than the same wafer, the second timing signalPIN is significantly affected by process variation. Such skew of thesecond timing signal PIN reduces a timing margin, resulting in thereduction in effective data area such as data eye.

One problem with typical stacked semiconductor apparatus isdeterioration of operation due to skew. As well as the second timingsignal PIN, skew for internal signals of each slave chip Slavedeteriorates the operation characteristics of a stacked semiconductormemory apparatus. Furthermore, timing margins of various internalsignals have been gradually reduced with the high speed operation of asemiconductor memory apparatus. In this regard, there has beenincreasing demand for a stacked semiconductor memory apparatus capableof correcting the skew for internal signals of each slave chip Slave.

FIG. 2 is a detailed block diagram of the typical timing signalgeneration unit 100 illustrated in FIG. 1.

As mentioned above, the timing signal generation unit 100 included ineach slave chip Slave generates internal timing signals required forread and write operations thereof. The internal timing signals mayinclude a first application signal YI, a second application signal BWEN,a third application signal IOSTBP, and a second timing signal PIN. Thefirst application signal YI is used to control an electrical connectionbetween segment input/output lines and bit lines and bit bar lines in aread or write operation. The second application signal BWEN is used tocontrol an electrical connection between input/output lines differentfrom each other in a write operation. The third application signalIOSTBP is used to control an electrical connection between linesdifferent from each other in a read operation. The second timing signalPIN is outputted from the final terminal of the timing signal generationunit 100 and includes the synchronization information required when theplurality of slave chips Slave transmit data to the master chip asmentioned above. The timing signal generation unit 100 includes aplurality of delay circuits 110, 120, 130 and 140. If the first timingsignal AYP is received, output units of the plurality of delay circuits110, 120, 130 and 140 output the first application signal YI, the secondapplication signal BWEN, the third application signal IOSTBP, and thesecond timing signal PIN, respectively. As mentioned above, the secondtiming signal PIN is outputted from the final terminal of the delaycircuit constituting the timing signal generation unit 100. This meansthat many transistors may exist from the reception of the first timingsignal AYP to the generation of the second timing signal PIN, ascompared with the number of the first application signal YI, the secondapplication signal BWEN and the third application signal IOSTBP, andthus the largest skew due to the PVT variation occurs in the secondtiming signal PIN.

A third timing signal Pre_AYP illustrated in FIG. 1 is transmitted fromthe master chip to the plurality of slave chips Slave, is advanced ascompared with the first timing signal AYP, and includes reception timinginformation of an address signal (not shown) transmitted from the masterchip to the plurality of slave chips Slave.

SUMMARY

Accordingly there is a need for an improved semiconductor apparatus thatmay obviate the above-mentioned problem. It should be understood,however, that some aspects of the invention may not necessarily obviatethe problem.

In the following description, certain aspects and embodiments willbecome evident. It should be understood that these aspects andembodiments are merely exemplary, and the invention, in its broadestsense, could be practiced without having one or more features of theseaspects and embodiments.

In one exemplary embodiment, a semiconductor apparatus may include: areference delay value check unit configured to receive a source signaland delay the source signal to generate a reference delay signal; aprocess delay value check unit configured to receive the source signaland delay the source signal to generate a process delay signal; and asignal generation unit configured to receive the reference delay signaland the process delay signal, receive an input signal, and variablydelay the input signal based on the reference delay signal and theprocess delay signal to generate an output signal.

In another exemplary embodiment, a method for delaying a signal of asemiconductor apparatus may include the steps of: comparing a referencedelay value with a process delay value; and variably delaying an inputsignal based on a comparison result obtained by comparing the referencedelay value with the process delay value, thereby generating an outputsignal.

In another exemplary embodiment, a stacked semiconductor memoryapparatus may include: a master chip; and a plurality of slave chips,wherein the plurality of slave chips are configured to commonly receivean input signal from the master chip, and include variable delay unitsconfigured to receive a reference delay value and a process delay valueto generate an output signal by variably delaying the input signal basedon the reference delay value and the process delay value, respectively.

In another exemplary embodiment, a method for generating a signal of astacked semiconductor memory apparatus may include the steps of:transmitting by a master chip a first timing signal to a plurality ofslave chips; comparing by each slave chip a reference delay value with aprocess delay value; and generating by each slave chip a delay timingsignal by variably delaying the first timing signal based on a resultobtained by comparing the reference delay value with the process delayvalue.

In another exemplary embodiment, a stacked semiconductor memoryapparatus may include: a master chip; and a plurality of slave chips,wherein the plurality of slave chips are configured to receive an inputsignal from the master chip, and wherein each slave chip includes: afirst delay circuit having a delay value unrelated to a process; asecond delay circuit having a delay to value related to the process; anda delay compensation circuit configured to compare the delay valueunrelated to the process with the delay value related to the process andcompensate for a delay value of the input signal.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings, which are incorporated in and constitute apart of this specification, explain various embodiments consistent withthe invention and, together with the description, serve to explain theprinciples of the invention.

FIG. 1 is a schematic diagram of a typical stacked semiconductor memoryapparatus using a TSV method;

FIG. 2 is a detailed block diagram of the typical timing signalgeneration unit illustrated in FIG. 1;

FIG. 3 is a schematic diagram of a stacked semiconductor memoryapparatus according to an exemplary embodiment;

FIG. 4 is a detailed block diagram of the timing signal generation unitand the variable delay unit illustrated in FIG. 3; and

FIG. 5 is a circuit diagram of the variable delay unit illustrated inFIGS. 3 and 4.

DETAILED DESCRIPTION

Reference will now be made in detail to the exemplary embodimentsconsistent with the present disclosure, examples of which areillustrated in the accompanying drawings. Wherever possible, the samereference characters will be used throughout the drawings to refer tothe same or like parts.

An exemplary stacked semiconductor memory apparatus variably delays afirst timing signal AYP based on PVT variation applied to each slavechip Slave, thereby correcting skew which may occur in each slave chipSlave.

FIG. 3 is a schematic diagram illustrating a stacked semiconductormemory apparatus according to an exemplary embodiment of the invention.In the stacked semiconductor memory apparatus illustrated in FIG. 3, avariable delay unit 200 is further provided between a reception terminalt1 of the first timing signal AYP, which is transmitted from a masterchip to a plurality of slave chips Slave, and a timing signal generationunit 100,

The variable delay unit 200 may be configured to detect PVT variation ineach slave chip Slave, generate a delay timing signal AYP1 by variablydelaying the first timing signal AYP based on the PVT variation, andsupply the timing signal generation unit 100 with the delay timingsignal AYP1.

The variable delay unit 200 may detect the PVT variation in each slavechip Slave before each slave chip Slave receives the first timing signalAYP. Accordingly, an operation in which the variable delay unit 200detects the PVT variation in each slave chip Slave may be performed inresponse to a signal (for example, a third timing signal Pre_AYP) whichis activated before the first timing signal AYP.

The variable delay unit 200 may supply the timing signal generation unit100 with the delay timing signal AYP1 obtained by variably delaying thefirst timing signal AYP based on the PVT variation, so that the timingsignal generation unit 100 included in each slave chip Slave maygenerate a second timing signal PIN which is activated at a targettiming. That is, skew of internal timing signals including the secondtiming signal PIN can be reduced. The skew of the internal timingsignals of the plurality of slave chips Slave can be reduced through thevariable delay units 200, so that a timing margin in the process oftransmitting signals of the stacked semiconductor memory apparatus canbe increased, thereby providing advantages in the high speed operationof the stacked semiconductor memory apparatus. In addition, it ispossible to reduce the necessity that the plurality of slave chips Slaveshould use a plurality of paths for transmitting the second timingsignal PIN because the plurality of paths are not integrated into asingle path due to the excessive skew of the internal timing signals. Ina stacked semiconductor memory apparatus using the TSV method, thereduction in the demand for unnecessary paths may be advantageous interms of the layout, area and degree of integration.

FIG. 4 is a detailed block diagram illustrating the timing signalgeneration unit 100 and the variable delay unit 200 illustrated in FIG.3. As illustrated in FIG. 3, the variable delay unit 200 may be includedin each slave chip Slave and coupled between a reception terminal of thefirst timing signal AYP and an input terminal of the timing signalgeneration unit 100. As mentioned above, the variable delay unit 200 maybe configured to detect the PVT variation in a corresponding slave chipSlave and supply the timing signal generation unit 100 with the delaytiming signal AYP1 obtained by variably delaying the first timing signalAYP based on the PVT variation. The timing signal generation unit 100may have a configuration substantially equal to the timing signalgeneration unit 100 illustrated in FIG. 2. Since the timing signalgeneration unit 100 may operate similarly to the timing signalgeneration, unit 100 illustrated in FIG. 2, except that the timingsignal generation unit 100 illustrated in FIG. 2 receives the firsttiming signal AYP but the timing signal generation unit 100 receives thedelay timing signal AYP1 from the variable delay unit 200, detaileddescription thereof will be omitted.

FIG. 5 is a circuit diagram illustrating the variable delay unit 200illustrated in FIGS. 3 and 4.

The variable delay unit 200 is configured to detect the PVT variation ina corresponding slave chip Slave and generate an output signal ‘out’ byvariably delaying an input signal ‘in’ based on the PVT variation. InFIGS. 3 and 4, the input signal ‘in’ corresponds to the first timingsignal AYP and the output signal ‘out’ corresponds to the delay timingsignal AYP1.

The detection of the PVT variation in the corresponding slave chip Slavemay be performed by comparing a reference delay value with a processdelay value. More precisely, the variable delay unit 200 is configuredto delay a source signal ‘source’ through delay circuits with twoconfigurations. The delay circuits with the two configurations mayinclude circuits in which a delay value is variously changed based onthe PVT variation. In general, in a semiconductor memory apparatus, adelay circuit generally may have two configurations. One is a delaycircuit using RC delay and the other one is a delay circuit using aplurality of inverters. The delay circuit using the RC delay has a delaytime which is proportional to the multiplication of resistance andcapacitance, and the delay circuit using the plurality of inverters hasa delay time based on the current amount of transistors constituting theinverters and the number of the inverters. Accordingly, the delay timeof the delay circuit using the plurality of inverters is significantlyaffected by the operation characteristics of the transistors. The delaytime of the delay circuit using the RC delay may be relativelyinsensitive (such as, for example, about 30%) to the PVT variation, ascompared with the delay circuit using the plurality of inverters. Inthis regard, the delay circuit using the RC delay and the delay circuitusing the plurality of inverters are suitable for the delay circuitswith the two configurations of the variable delay unit 200. The delaytime of the delay circuit using the RC delay having delay time variationrelatively insensitive to the. PVT variation will be referred to as areference delay value (or a delay value unrelated to a process), and thedelay time of the delay circuit using the plurality of inverters havingdelay time variation relatively sensitive to the PVT variation, ascompared with the delay circuit using the RC delay, will be referred toas a process delay value or a delay value related to a process). Thedelay circuits with the two configurations may include all delaycircuits having different delay time variations based on the PVTvariation. However, it should be noted that the fact that the delaycircuits (refer to reference numerals 210 and 220 of FIG. 5) with thetwo configurations includes the delay circuit using the RC delay and thedelay circuit using the plurality of inverters does not limit tonecessary elements for realizing the invention.

As illustrated in FIG. 5, the variable delay unit 200 may include areference delay value check unit 210, a process delay value check unit220, and a signal generation unit 230.

The reference delay value check unit 210 may be configured to delay thesource signal ‘source’ and generate a reference delay signal d1. Asillustrated in FIG. 5, the reference delay value check unit 210 mayinclude a delay circuit having a resistor R and a capacitor C.

The process delay value check unit 220 may be configured to delay thesource signal ‘source’ and generate a process delay signal d2. Asillustrated in FIG. 5, the process delay value check unit 220 mayinclude a delay circuit having a plurality of inverters IV.

The signal generation unit 230 may be configured to variably delay theinput signal ‘in’ based on the reference delay signal d1 and the processdelay signal d2 and generate the output, signal ‘out’.

The source signal ‘source’ may use a signal activated earlier ascompared with the input signal ‘in’. In FIGS. 3 and 4, the source signal‘source’ may correspond to the third timing signal Pre_AYP. In. FIGS. 3and 4, it should be noted that the invention is not limited to the useof the third timing signal Pre_AYP as the source signal ‘source’ as aprerequisite element for realizing the invention.

The reference delay value check unit 210 and the process delay valuecheck unit 220 may receive and delay the source signal ‘source’ in thesame manner. Consequently a difference between the delay time of thereference delay signal d1 and the delay time of the process delay signald2, which are generated by the reference delay value check unit 210 andthe process delay value check unit 220, may be information indicatingthe degree by which a corresponding slave chip Slave has been affectedby the PVT variation. When the delay time of the process delay signal d2is shorter than the delay time of the reference delay signal d1, it maymean that transistors of the corresponding slave chip Slave operate at ahigh speed. However, when the delay time of the process delay signal d2is longer than the delay time of the reference delay signal d1, it maymean that transistors of the corresponding slave chip Slave operate at alow speed. Consequently, while the variable delay unit 200 variablydelays the input signal ‘in’ to generate the output signal ‘out’, thevariable delay unit 200 may delay the input signal ‘in’ by a first delaytime to output the output signal ‘out’ when the delay time of theprocess delay signal d2 is longer than the delay time of the referencedelay signal d1, and delay the input signal ‘in’ by a second delay timelonger than the first delay time to output the output signal ‘out’ whenthe delay time of the process delay signal d2 is shorter than the delaytime of the reference delay signal d1.

The signal generation unit 230 may include a delay signal generationsection 231, a selection signal generation section 232, and is a signaloutput section 233.

The delay signal generation section 231 may be configured to delay theinput signal ‘in’ by the first delay time to generate a first delaysignal ind1, and delay the input signal ‘in’ by the second delay time togenerate a second delay signal ind2. The delay signal generation section231 may include a first delay circuit 2311 and a second delay circuit2312. The first delay circuit 2311 delays the input signal ‘in’ by thefirst delay time to generate the first delay signal ind1 and the seconddelay circuit 2312 delays the input signal ‘in’ by the second delay timeto generate the second delay signal ind2. The first delay circuit 2311and the second delay circuit 2312 may include general delay circuits,respectively.

The selection signal generation section 232 may be configured to comparethe reference delay signal d1 with the process delay signal d2 andgenerate a selection signal sel. As illustrated in FIG. 5, the selectionsignal generation section 232 may include a pass gate 2321, an inverter2322, a latch circuit 2323, and an inverter 2324. The inverter 2322inverts the reference delay signal d1. The pass gate 2321 allows theprocess delay signal d2 to pass therethrough in response to thereference delay signal d1 and an output signal of the inverter 2322. Thelatch circuit 2323 latches an output signal of the pass gate 2321. Theinverter 2324 inverts an output signal of the latch circuit 2323 andoutput the inverted output signal as the selection signal sel. Theselection signal generation section 232 configured as illustrated inFIG. 5 inverts a logic value of the process delay signal d2 to latch theprocess delay signal d2 as the selection signal sel when the referencedelay signal d1 falls to a low level from a high level. Thus, when theprocess delay signal d2 has a high level pulse width longer than that ofthe reference delay signal d1, the selection signal generation section232 latches the selection signal sel at a low level. However, when theprocess delay signal d2 has a high level pulse width shorter than thatof the reference delay signal d1, the selection signal generationsection 232 latches the selection signal sel at a high level.

The signal output section 233 may be configured to select one of thefirst delay signal ind1 and the second delay signal ind2 in response tothe selection signal sel and output the selected signal as the outputsignal ‘output’. As illustrated in FIG. 5, the signal output section 233may include an inverter 2331, a pass gate 2332, and a pass gate 2333.The inverter 2331 inverts and outputs the selection signal sel. The passgate 2332 allows the first delay signal ind1 to pass therethrough inresponse to an output signal of the inverter 2331 and the selectionsignal sel and outputs the first delay signal ind1 as the output signal‘output’. The pass gate 2333 allows the second delay signal ind2 to passtherethrough in response to the selection signal sel and the outputsignal of the inverter 2331 and outputs the second delay signal ind2 asthe output signal ‘output’. The signal output section 233 illustrated inFIG. 5 outputs the second delay signal ind2 as the output signal ‘out’when the selection signal sel is at a high level, and outputs the firstdelay signal ind1 as the output signal ‘out’ when the selection signalsel is at a low level.

The stacked semiconductor memory apparatus illustrated in FIGS. 3 to 5according to an exemplary embodiment is characterized in that each slavechip Slave variably delays the first timing signal AYP based on the PVTvariation to correct the skew of internal signals thereof. Suchcharacteristics are not limited to the semiconductor memory apparatus.The invention can be applied to a stacked semiconductor apparatus.

Furthermore the stacked semiconductor memory apparatus illustrated inFIGS. 3 to 5 according to an embodiment is characterized in that thedegree of influence by the PVT variation can be understood by checkingthe reference delay value and the process delay value. Suchcharacteristics are not limited to the stacked semiconductor memoryapparatus. Compensating for the influence by the PVT variation bychecking the reference delay value and the process delay value can beapplied to all types of semiconductor memory apparatuses, as well as thestacked semiconductor memory apparatus.

The variable delay unit 200 illustrated in FIG. 5 according to anembodiment outputs one of the first delay signal ind1 and the seconddelay signal ind2 as the delay timing signal AYP1 based on a resultobtained by comparing the reference delay value with the process delayvalue. As illustrated in FIG. 5, since the configuration in which thedelay timing signal AYP1 has two delay times is set for the purpose ofconvenience, it may be possible to employ a configuration in which thedelay timing signal AYP1 has three or more delay times as is required,that is, the delay time can be more precisely adjusted. It should benoted that the configuration in which the delay timing signal AYP1 hastwo steps of delay times as illustrated in FIG. 5 is only exemplary, andthe invention is not limited thereto.

In the stacked semiconductor memory apparatus illustrated in FIG. 3 themaster chip and the plurality of slave chips Slave are electricallyconnected to each other through the TSV method. However, the inventionis not limited to the stacked semiconductor memory apparatus using theTSV method. For example, the invention can be applied to various typesof multichip semiconductor memory apparatuses using a SIP method, a POPmethod and the like.

While certain embodiments have been described above, it will beunderstood to those skilled in the art that the embodiments describedare by way of example only. Accordingly, the semiconductor apparatus,the method for delaying a signal thereof, the stacked semiconductormemory apparatus, and the method for generating a signal thereofdescribed herein should not be limited based on the describedembodiment. Rather the semiconductor apparatus, the method for delayinga signal thereof, the stacked semiconductor memory apparatus, and themethod for generating a signal thereof described herein should only belimited in light of the claims that follow when taken in conjunctionwith the above description and accompanying drawings.

What is claimed is:
 1. A semiconductor apparatus comprising: a referencedelay value check unit configured to receive a source signal and delaythe source signal to generate a reference delay signal, wherein thereference delay value check unit comprises a delay circuit for delayingthe source signal, and the delay circuit performs a delay operationusing RC delay; a process delay value check unit configured to receivethe source signal and delay the source signal to generate a processdelay signal, wherein the process delay value check unit comprises adelay circuit for delaying the source signal, and the delay circuitperforms a delay operation using a plurality of inverters; and a signalgeneration unit configured to receive the reference delay signal and theprocess delay signal, receive an input signal, and variably delay theinput signal based on the reference delay signal and the process delaysignal to generate an output signal.
 2. The semiconductor memoryapparatus according to claim 1, wherein the input signal is delayed by afirst delay time and is outputted as the output signal when a delay timeof the process delay signal is longer than a delay time of the referencedelay signal, the input signal is delayed by a second delay time and isoutputted as the output signal when the delay time of the process delaysignal is shorter than the delay time of the reference delay signal, andthe first delay time is shorter than the second delay time.
 3. Thesemiconductor memory apparatus according to claim 2, wherein the signalgeneration unit comprises: a delay signal generation section configuredto variably delay the input signal by the first delay time to generate afirst delay signal and variably delay the input signal by the seconddelay time to generate a second delay signal; a selection signalgeneration section configured to compare the process delay signal withthe reference delay signal and generate a selection signal; and a signaloutput section configured to select one of the first delay signal andthe second delay signal in response to the selection signal and output aselected signal as the output signal.
 4. A method for delaying a signalof a semiconductor apparatus, comprising the steps of: comparing areference delay value with a process delay value wherein comparing thereference delay value with the process delay value comprises: generatinga reference delay signal by delaying a source signal using RC delay;generating a process delay signal by delaying the source signal using aplurality of inverters; and comparing the reference delay signal withthe process delay signal; and variably delaying an input signal based ona comparison result obtained by comparing the reference delay value withthe process delay value, thereby generating an output signal.
 5. Themethod according to claim 4, wherein, in the step of generating theoutput signal, the input signal is delayed by a first delay time and isgenerated as the output signal when the process delay value is largerthan the reference delay value based on the comparison result, the inputsignal is delayed by a second delay time and generated as the outputsignal when the process delay value is smaller than the reference delayvalue based on the comparison result, and the first delay time isshorter than the second delay time.
 6. The method according to claim 4,wherein the step of generating the output signal comprises the steps of:generating a first delay signal by delaying the input signal by a firstdelay time; generating a second delay signal by delaying the inputsignal by a second delay time shorter than the first delay time; andselecting one of the first delay signal and the second delay signalbased on the comparison result, thereby generating a selected signal asthe output signal.